Differential amplifiers are widely used in analog and digital circuits to amplify a differential voltage between two input signals. Ideally, a differential amplifier amplifies only the difference between the input signals while rejecting common-mode input changes such as noise. Differential amplifiers have found important applications where signals are contaminated by noise signals. For example, digital signals transmitted over a long cable may pick up miscellaneous noise signals during the signal transmission. The differential amplifiers reject the noise signals while amplifying the digital signals, thereby leading to the recovery of the original signals.
Unfortunately, conventional differential amplifiers present several drawbacks. For example, fully differential amplifier circuits often suffer from limited linear operating ranges as described in U.S. Pat. No. 5,289,136 by DeVeirman et al., the disclosure of which is incorporated herein in its entirety. As a result, the differential amplifier circuits can only receive a narrow range of input voltages to produce a linear output. If the input voltages venture outside of the narrow input voltage range, the differential amplifier circuits produce a non-linear output.
In addition, the gain of conventional differential amplifiers may vary in response to a change in temperature. This is because base-emitter voltages of individual transistors in the differential amplifiers are highly sensitive to temperature variation. Since base-emitter voltages affect the transconductance of a transistor, the gain of the differential amplifier may not be constant or linear when such temperature variation occurs. In such cases, the gain of the conventional differential amplifiers may not be predictable due to the temperature variations.
Furthermore, the conventional differential amplifiers may not provide sufficient gain for today's state of the art low current or low voltage applications. For example, the state of the art high speed analog or digital circuits often employ low currents and/or low voltages to speed up the operation of the circuit while reducing power requirements for continuously decreasing die sizes. At such low currents and/or voltages, the conventional differential amplifiers may not provide a large enough gain to operate properly in high speed applications.
Thus, what is needed is a differential amplifier that provides a large and predictable linear gain over a wide input range. What is also needed is a differential amplifier that provides such linear gain even in low current and low voltage applications.